Hello, thanks for using health care magic. If your wife has a regular twenty eight day cycle then her most probable date of ovulation would have been the nineteenth which is on day fourteen of her cycle that started on the 6th. However, when it comes to the human body nothing can be so exact because the human body is not robotic but adapts to environmental and physiological/metabolic changes. Simple stress could delay the ovulation and even for some unknown reasons, ovulation could occur a day or two before or after the expected day of ovulation. Thus any unprotected intercourse 4 days before or after the fourteenth day of menses is likely to lead to pregnancy. Implantation occurs between day 6 to day 12 after fertilization. Thus adding 6 to12 days to the 19th of march gives us that the probable period of implantation was between 25th and 1st which is similar to what you have. An early ultrasound gives a appreciable value for age of pregnancy but does not tell with certitude the exact date of fertilization or implantation. Best regards Dr Achuo